Motor-driven wheel driving apparatus has been proposed to improve the driving efficiency in cases of driving a wheel by an electric motor. Additionally, the apparatus is used to drive a wheel, e.g. of an electric vehicle, where the wheel is directly driven by the electric motor. However, such motor-driven wheel driving apparatus requires a large torque for an electric motor. Thus, it is necessary to use a large scale motor with high power. This not only increases manufacturing cost but weight of the vehicle and accordingly makes it difficult to ensure sufficient driving performance.
Additionally, motor-driven wheel driving apparatus (an in-wheel motor) equipped with a reduction gear mechanisms have been proposed. Here, an electric motor and a planetary reduction gear are arranged within a space inside a wheel. The rotational output of the electric motor is transmitted to the wheel via the planetary reduction gear mechanism (see e.g. Japanese Patent Publication No. 3440082).
When adopting the planetary reduction gear mechanism as the reduction gear mechanism, its output shaft to transmit the rotational output of the electric motor to the wheel, via the planetary reduction gear mechanisms, has to take out its output with coinciding its axial center to that of the motor shaft after once having been divided in the axial direction of the motor-driven wheel driving apparatus. Accordingly, problems exist in such an apparatus in that its structure is difficult to assemble and the strength against inclination of the wheel during running of the vehicle is low due to the supporting span of the output shaft must be reduced.
In order to solve these problems, a known motor-driven wheel driving apparatus is shown in FIG. 8. The motor-driven wheel driving apparatus 51 has an electric motor 54 and a reduction gear mechanism 55 inside a wheel 53 on which a tire 52 is mounted. The wheel 53 can be driven by rotational output of the electric motor 54.
The electric motor 54 has a stator 57 secured to a side of a case 56 arranged inside the wheel 53. A rotor 58 is arranged opposite to the stator 57. An output shaft 59, within the rotor, transmits the rotational output of the rotor 58 to the wheel 53 via the reduction gear mechanism 55. The stator 57 and the rotor 58 are secured to a side of the case 56 and are sandwiched by covers 60 and 61 to form the electric motor 54.
One end 59a of the output shaft 59 is integrally formed with a mounting flange 62. The wheel 53 is secured, via hub bolts 63, to the flange 62. The end 59a of the output shaft 59 is rotatably supported by a rolling bearing 64 within a shaft insert aperture 56b of the case 56. The other end 59b is also rotatably supported by a rolling bearing 65 within a central recess 60a of the outer cover 60.
The reduction gear mechanism 55, within the case 56, is formed by a plurality of gears 55a, 55b, 55c and 55d. A first gear 55a is arranged coaxially and integrally formed with the end of the rotor 58. Second and third gears 55b and 55c are secured on the same supporting shaft 66 and thus rotate simultaneously. The second gear 55b and the first gear 55a mesh with each other. One end 66a of the supporting shaft 66 is rotatably supported by a rolling bearing 67 within a recess 61a in the inner cover 61. The other shaft end 66b is also rotatably supported by a rolling bearing 68 within a recess 56a of the case 56. The force gear 55d is secured on the output shaft 59 and meshes with the third gear 55c. 
According to such a structure, the output shaft 59 of the electric motor 54 is rotatably supported at opposite ends of the case 56. The shaft 59 is passed through the aperture center of rotation of the fourth gear 55d, which is a final stage of the reduction gear means 55. Also, the shaft 59 is passed through the shaft inserting aperture 58a of the rotor 58. Thus, the apparatus can be easily assembled by sequentially fitting the structural parts of the reduction gear mechanism 55, the inner cover 61, the structural parts of the electric motor 54, and the outer cover 60 onto the output shaft 59 by using it as a reference part. In addition since the output shaft 59 is supported at substantially opposite ends of the motor-driven wheel driving apparatus 51, it is possible to ensure the supporting span of the output shaft 59. Thus, it is possible to obtain a sufficient supporting strength against the inclination of wheel during running of the vehicle (see Japanese Laid-open Patent Publication No. 81436/1995).
In such a motor-driven wheel driving apparatus, while the reduction gear mechanism 55 can easily assemble various structural parts, the installation space for the rolling bearings 64 and 65 for supporting the output shaft 59 is limited. This is due to the electric motor 54 of high rotational output requiring a large mounting space. Accordingly, the loading capacity against the inclination of the wheel 53 during running of the vehicle, against the moment load, becomes insufficient. Thus, it has become necessary to improve the durability of the rolling bearings 64 and 65.